Method and apparatus for partial and course frequency offset estimation in a digital audio broadcasting (DAB) system

Abstract

A method and apparatus are disclosed for frequency offset estimation in a hybrid in-band on-channel (HIBOC) digital audio broadcasting (DAB) system. The frequency offset estimation algorithm first determines the coarse frequency offset, in terms of integer number of OFDM bin separations between an actual and measured location of a correlated peak, followed by estimation and tracking of the partial (residual) offset in a continuous fashion. A frame is correlated at a first frequency, and then the frequency is shifted by a predefined amount, such as half of the inter-bin frequency amount, Deltaf, before correlating again. The measurement with the highest peak of the plurality of frequency values is utilized to obtain the course frequency offset estimate. The partial frequency offset estimation algorithm utilizes phase information contained in reference frequency points in the frequency domain. The phase rotation of the reference vector is proportional to the frequency error, and the frequency error is extracted and filtered in the time domain. The initial partial frequency offset estimation is corrected and used for continuous frequency tracking. Following the course frequency offset estimation, the unmodulated pilot bins (and all other bins), are within at least half the of the OFDM bin separation. In the presence of a frequency offset, the complex bins start rotating. The rate of the rotation is a function of the extent of the partial frequency offset. The change in phase from one frame to the next is proportional to the rate of rotation, and the sign of the rotation indicates the direction of the shift.

We claim:  
     
       1. A method for coarse frequency offset estimation in an OFDM communication system, comprising the steps of: 
       receiving a digital signal containing a signature sequence;  
       correlating said received digital signal using at least two frequencies; and  
       selecting one of said frequencies that provides a correlated peak having the largest magnitude to position said correlated peak near a bin.  
     
     
       2. The method of  claim 1 , further comprising the step of estimating the coarse frequency offset by determining whether a correlated peak associated with said signature sequence is in an expected location. 
     
     
       3. The method of  claim 1 , wherein said signature sequence is a Barker code. 
     
     
       4. The method of  claim 1 , wherein said signature sequence provides reference information contained in discrete known frequency points in the frequency domain. 
     
     
       5. The method of  claim 1 , further comprising the step of applying a calculated coarse frequency offset to a forward correction mechanism. 
     
     
       6. The method of  claim 1 , further comprising the step of applying a calculated coarse frequency offset to a feedback mechanism. 
     
     
       7. A method for estimating the partial frequency offset in an OFDM communication system, comprising the steps of: 
       estimating a coarse frequency offset to position a correlated peak associated with a signature sequence near an OFDM bin within a predefined distance of an OFDM bin separation;  
       evaluating a change in phase of an unmodulated bin over at least two frames; and  
       estimating the partial frequency offset based on said change in phase.  
     
     
       8. The method of  claim 7 , wherein said coarse frequency offset estimation positions said correlated peak half of an OFDM bin separation. 
     
     
       9. The method of  claim 7 , further comprising the steps of extracting and filtering said partial frequency offset in the time domain. 
     
     
       10. The method of  claim 7 , further comprising the step of correcting and using said partial frequency offset estimate for continuous frequency tracking. 
     
     
       11. The method of  claim 7 , wherein the extent of the frequency offset is a function of a rate of rotation of said change in phase. 
     
     
       12. The method of  claim 7 , further comprising the step of making said rate of rotation equal to zero. 
     
     
       13. The method of  claim 7 , wherein a polarity of a rate of rotation of said change in phase indicates the direction of a frequency offset. 
     
     
       14. A receiver for receiving a digital signal containing a signature sequence in an OFDM communication system, comprising: 
       a filter matched to said signature sequence for correlating said received digital signal using at least two frequencies; and  
       means for selecting one of said frequencies that provides a correlated peak having the largest magnitude to position said correlated peak near a bin.  
     
     
       15. The receiver of  claim 14 , further comprising means for estimating the coarse frequency offset by determining whether a correlated peak associated with said signature sequence is in an expected location. 
     
     
       16. The receiver of  claim 14 , wherein said signature sequence is a Barker code. 
     
     
       17. The receiver of  claim 14 , wherein said signature sequence provides reference information contained in discrete known frequency points in the frequency domain. 
     
     
       18. The receiver of  claim 14 , further comprising a forward correction mechanism for compensating for a calculated coarse frequency offset. 
     
     
       19. The receiver of  claim 14 , further comprising a feedback mechanism for compensating for a calculated coarse frequency offset. 
     
     
       20. A receiver in an OFDM communication system, comprising: 
       a coarse frequency offset estimator to position a correlated peak associated with a signature sequence near an OFDM bin within a predefined distance of an OFDM bin separation;  
       a phase estimator for evaluating a change in phase of an unmodulated bin over at least two frames; and  
       means for estimating a partial frequency offset based on said change in phase.  
     
     
       21. The receiver of  claim 20 , wherein said coarse frequency offset estimator positions said correlated peak half of an OFDM bin separation. 
     
     
       22. The receiver of  claim 20 , further comprising a loop filter for extracting and filtering said partial frequency offset in the time domain. 
     
     
       23. The receiver of  claim 20 , wherein the extent of the frequency offset is a function of a rate of rotation of said change in phase. 
     
     
       24. The receiver of  claim 20 , further comprising a feedback loop to make said rate of rotation equal to zero. 
     
     
       25. The receiver of  claim 20 , wherein a polarity of a rate of rotation of said change in phase indicates the direction of a frequency offset. 
     
     
       26. The receiver of  claim 20 , wherein the partial frequency offset is estimated as follows:            Δf   res     =       ΔΘ   ·     f   fr         2      π         ,                   
       where Δf res  is the partial frequency offset ΔΘ is the inter-frame phase rotation and f fr  is the frame rate.